Various types of circuits have often used pulse width modulation (PWM) signals in recent years. For example, a switching power supply circuit performs a switching operation using a PWM signal. At this time, the switching power supply circuit can control the voltage level of an output voltage generated thereby by changing the pulse width of the PWM signal. In using the PWM signal, the switching power supply circuit can shape the rising waveform of the output voltage by performing soft-start control to set the initial duty ratio of the PWM signal to a small value and then increasing the duty ratio with a lapse of time.
There have been proposed many such PWM signal generation circuits, which generate a PWM signal using soft-start control. Patent Literature 1 discloses an example of such a PWM signal generation circuit. A method for soft-starting a switching power supply disclosed in Patent Literature 1 includes gradually increasing the duty ratio of a PWM signal to be generated while monitoring the voltage level of an output voltage VOUT of the switching power supply circuit. However, the method for soft-starting a switching power supply disclosed in Patent Literature 1 involves always calculating the difference between a reference voltage Vref and the output voltage VOUT. Accordingly, assuming that a processor calculates this difference through a computing process, the processor is disadvantageously required to have a high computing capacity.
In view of the foregoing, Patent Literatures 2 and 3 disclose PWM signal generation circuits that soft-start control a PWM signal without having to monitor the output voltage of the circuit to be controlled, such as a switching power supply circuit.
Patent Literature 2 generates a PWM signal by providing various setting signals to a logic circuit (FIGS. 1 to 3 of Patent Literature 2). At this time, Patent Literature 2 performs soft-start control using a circuit shown in FIG. 7 of Patent Literature 2. Immediately after starting operating, the circuit shown in FIG. 7 of Patent Literature 2 sets the output value of DFF5 to “0H” and sets a signal EN1 to “L.” Since the output value is “0H” and a signal PM1ONS is “L”, the output of a gate 612 is set to “H,” and the output value “0H” of DFF5 is changed to “1H” and sent to CB15. Computation similar to that when the on width is “1H” is performed in the apparatus. While a signal PM1 is changed from “L” to “H,” the signal EN1 is “L.” Accordingly, the signal PWM1 is kept at “L” by a gate 205. Since the signal PM1ONS is “H” and the signal RDWN1 is “L,” the output value of DFF5 is inputted to CB15 while remaining “0H.” Subsequently, the register value of DFF5 is updated to “1H” through predetermined computation. The signal EN1 of an on width output control unit 61 is also changed to “H”, so that a PWM signal is outputted.
Patent Literature 3 discloses a soft-start circuit of a switching power source. This switching power source includes a triangular wave generating circuit, an error amplifier, and a PWM comparator. Normally, the switching power source obtains PWM pulses by comparing a triangular wave amplitude output of the triangular wave generating circuit with an output voltage of the error amplifier serving as a reference (comparison) voltage by using the PWM comparator. The soft-start circuit of the switching power source includes a soft-start reference value setting part and a counting circuit. The soft-start reference value setting part includes resistance networks and switches and uses the same means as an upper/lower limit setting part that includes a network and switch for setting upper and lower limits of the amplitude of a triangular wave generated by the triangular wave generating circuit when power is turned on. The counting circuit counts cycles of a triangular wave generated by the triangular wave generating circuit to obtain any multiple lengths of soft-start time in order to switch among the switches.